Adrenocorticotropin (ACTH), a 39 amino acid peptide, is produced by cleavage of a large precursor molecule, pro-opiomelanocortin (POMC). Post-translational enzymatic processing of POMC yields other biologically active peptides (e.g., corticotropin-like intermediate peptide (CLIP), melanocyte-stimulating hormone (MSH), and lipotrophin (LPH)) in addition to ACTH as a result of tissue-specific processing of POMC. See Bicknell, J. Neuroendocrinology 20: 692-99 (2008).
The POMC gene has been remarkably conserved throughout evolution. A variety of organisms have a single functional copy of the gene with the same overall gene structure. The POMC gene is predominantly expressed in the anterior and intermediate lobes of the pituitary, and it is generally accepted that the majority of POMC peptides found in the circulation are derived from the pituitary, whereas POMC peptides produced in extra-pituitary tissues (e.g., brain, lymphocytes, skin, testis, thyroid, pancreas, gut, kidney adrenal and liver) act in an autocrine or paracrine fashion. See Bicknell, J. Neuroendocrinology 20: 692-99 (2008).
POMC peptides, including ACTH, are believed to act primarily through melanocortin receptors (MCRs), a family of five G protein-coupled receptors (i.e., MC1R, MC2R, MC3R, MC4R and MC5R). MCRs are expressed in diverse tissues, and serve discrete physiological functions. MC1R, which is expressed on melanocytes, macrophages and adipocytes, is involved in pigmentation and inflammation. MC2R, which is expressed in the adrenal cortex, is involved in adrenal steroidogenesis. MC3R, which is expressed in the central nervous system (CNS), gastrointestinal (GI) tract and kidney, is involved in energy homeostasis and inflammation. MC4R, which is expressed in the CNS and spinal cord, is involved in energy homeostasis, appetite regulation and erectile function. MC5R, which is expressed on lymphocytes and exocrine cells, is involved in exocrine function and regulation of sebaceous glands. See Ramachandrappa et al., Frontiers in Endocrinology 4:19 (2013).
MC2R is reported to be unique among the MCR family for being highly specific for ACTH. See, Mountjoy K G et al., Science 1992; 257:1248-1251; and Schioth H B et al, Life Sci 1996; 59: 797-801. However, while MC3R is the only MCR with significant affinity for gamma-MSH, it can also bind alpha-MSH and ACTH with approximately equal affinity. See Gantz I, et al., J Biol Chem 1993; 268: 8246-8250. Also, at extremely high plasma concentrations, ACTH can bind to and activate MC1R resulting in hyperpigmentation, e.g., observed in subjects with familial glucocorticoid deficiency (FGD) (Turan et al., “An atypical case of familial glucocorticoid deficiency without pigmentation caused by coexistent homozygous mutations in MC2R (T152K) and MC1R (R160W).” J. Clin. Endocrinol. Metab. 97E771-E774 (2012)).
ACTH, one of the major end-products of POMC processing, is a hormone that is essential for normal steroidogenesis and the maintenance of normal adrenal weight. ACTH is secreted by the pituitary gland in response to physiological or psychological stress and its principal effects are increased production and release of corticosteroids. In particular, ACTH is secreted from corticotropes in the anterior lobe (or adenohypophysis) of the pituitary gland in response to the release of the hormone corticotropin-releasing hormone (CRH) by the hypothalamus. Once secreted, ACTH then travels to the adrenal cortex, where it binds to and activates MC2R. Activation of MC2R results in the production of cAMP in the adrenal cell. cAMP binds and activates protein kinase (PKA), which activates the conversion of the lipid cholesterol to the steroid hormone cortisol.
Cortisol is a hormone that affects numerous biological processes in order to restore homeostasis after stress. Exemplary processes regulated by cortisol include regulating glucose homeostasis, increasing blood pressure, gluconeogenesis, promoting metabolism of glycogen, lipids, and proteins, and suppressing the immune system. Under normal physiological conditions, cortisol levels are tightly regulated. However, in some conditions (including diseases and disorders further described herein), cortisol levels are elevated. The overproduction of cortisol has been shown to have many negative effects, such as damaging the hippocampus, a region of the brain that is critical for cognitive functions and regulation of the hypothalamus/pituitary/adrenal axis; increasing fat deposits, blood pressure levels, and blood sugar levels; bone loss; muscle weakness; and suppression of the immune system. Therefore, elevated cortisol levels may play a role in ACTH-driven hypercortisolism (such as Cushing's Disease or Cushing's Syndrome), obesity, diabetes, sleep apnea, adrenal hyperplasia (such as congenital adrenal hyperplasia), depression, anxiety disorders, cancer (such as Cushing's Syndrome resulting from ectopic ACTH expression, e.g., in small cell lung cancer, non-small cell lung cancer (NSCLC), pancreatic carcinoma, neural tumors, or thymoma), muscle atrophies, hypertension, cognitive dysfunction, galactorrhea and metabolic syndromes.
Aldosterone is a hormone released by the adrenal glands that helps regulate blood pressure. In particular, aldosterone increases the reabsorption of sodium and water and the release of potassium in the kidneys. In some disease conditions, aldosterone levels are elevated. For example, primary and secondary hyperaldosteronism occur when the adrenal gland releases too much of the hormone aldosterone. Primary hyperaldosteronism such as Conn's syndrome results from a problem with the adrenal gland itself that causes the release of too much aldosterone, whereas the excess aldosterone in secondary hyperaldosteronism is caused by something outside the adrenal gland that mimics the primary condition, e.g., by causing the adrenal gland to release too much aldosterone. Primary hyperaldosteronism used to be considered a rare condition, but some experts believe that it may be the cause of high blood pressure in some patients. Most cases of primary hyperaldosteronism are caused by a noncancerous (benign) tumor of the adrenal gland. The condition is most common in people ages 30-50 years. Secondary hyperaldosteronism is frequently due to high blood pressure and it may also be related to disorders such as cirrhosis of the liver, heart failure, and nephrotic syndrome. Therefore, elevated aldosterone levels may play a role in hyperaldosteronism including primary hyperaldosteronism (such as Conn's syndrome), secondary hyperaldosteronism and familial hyperaldosteronism.